The present invention relates to the mapping of data and pilot symbols in an OFDM-system.
Pilot symbols are necessary for a channel estimation such as to effect coherent detection of e.g. OFDM-modulated signals. The use of pilot symbols for channel estimation, however, introduces overhead and it is thus desirable to keep the number of pilot symbols to a minimum. The number of needed pilot symbols for a desired bit error rate and Doppler frequency is highly dependent on the propagation (channel) characteristics as well as the pilot pattern used in orthogonal frequency division multiplex (OFDM) systems.
A mobile channel introduces multipath distortion of the signaling wave forms. Both the amplitude and phase are corrupted and the channel characteristics changes because of movements of the mobile station.
In order to perform coherent detection, reliable channel estimates are required. This can be obtained by occasionally transmitting known data or so-called xe2x80x9cpilot symbolsxe2x80x9d. The receiver interpolates the channel information derived from the pilots to obtain the channel estimate for the data signal. Coherent modulation gives a good performance, but because of the necessary channel estimation, it requires more complexity at the receiver side.
Orthogonal frequency division multiplexing (OFDM) has been proposed recently for several broadcast systems and there is a growing interest in using the technique for the next generation of mobile communication systems. In OFDM systems the information signal can be seen as divided and transmitted by several narrow band sub-carriers. Typically, for practical OFDM systems, the frequency spacing is less than the coherence band width and the symbol time is less than the coherence time. This means that a receiver and pilot estimation pattern that take advantage of the relatively large coherence band width and coherence time can manage with less pilot symbols, thereby minimizing the overhead introduced by the pilot symbols.
The use of pilot symbols for a channel estimation in OFDM systems is for example disclosed in xe2x80x9cPilot Assisted Channel Estimation for OFDM in Mobile Cellular Systemsxe2x80x9d of Tufvesson and Maseng, Department of Applied Electronics, Lund University, Sweden, VTC-97.
Recently a mapping of OFDM symbols in GSM timeslots such as to provide compatibility of an OFDM transmission system with the current GSM system has been proposed as it is shown in FIG. 1. As can be seen from FIG. 1, according to this known proposal, two OFDM symbols S1, S2 with respectively a time duration of 240 microseconds together with a guard time of 48 microseconds between two adjacent OFDM symbols S1, S2 and having a time duration of respectively 48 microseconds fit into one GSM timeslot with the known time duration of approximately 576.9 microseconds.
The guard time of the known proposal with a time duration of 48 microseconds has been designed for a worst case scenario, i.e. for large multipath effects as is the case e.g. when the user is moving fast in outdoor applications. For indoor applications, however, the guard time of 48 microseconds usually is not necessary, and the minimum necessary guard time in indoor applications with a slow moving user can be as small as e.g. 5 microseconds. Therefore, the guard time of 48 microseconds unnecessarily reduces the effective bit rate of the transmission.
However, it is not possible solely to minimize the guard time and to maximize the time duration of the OFDM symbols S1, S2, as a change of the time duration of the OFDM symbols automatically implies a modification of the subcarrier spacing of the subcarriers of the OFDM system due to the known properties of such a system.
The object of the present invention is to provide a mapping of data in an OFDM system with an improved effective bit transmission rate, and which can find application particularly in (slow moving) indoor applications or in low-range outdoor applications.
This object is achieved by means of the features of the independent claims. The dependent claims develop further the idea of the invention.
The central idea of the invention thereby is to reduce the guard time to the minimum necessary time duration and to use the saved time slot to interpose a midamble between two succeeding OFDM symbols. To maintain the subcarrier spacing of the subcarriers of the OFDM system and at the same time to provide for a midamble time duration shorter than the time duration of the OFDM symbols (e.g. 240 microseconds), the time duration of the midamble MA equals to 1/n of the time duration of the OFDM symbols, n being an integer greater than 1.
According to the present invention therefore a wireless transmission method using an OFDM system is provided comprising a plurality of equally spaced subcarriers. The transmission is effected in timeslots. One timeslot comprises two symbols having the same time duration. A midamble is introduced between respectively two OFDM-modulated symbols. The time duration of the midamble thereby corresponds to 1/n of the time duration of the symbols, n being an integer greater than 1.
The transmission of the midamble can be effected according to the OFDM system and only every n-th subcarrier can be modulated when transmitting the midamble.
The transmission of the midamble can be effected according to a single carrier system still maintaining OFDM-modulated symbols adjacent to the midamble.
The midamble can be used to transmit pilot symbols such as to effect the channel estimation for a coherent transmission.
A known training sequence can be used to create the pilot symbols by a single carrier modulation. The channel estimation can be performed by comparing the time to frequency transform of the known single carrier training sequence with a time to frequency transform of a received training sequence.
A channel estimation can be performed based on a received pilot symbol after effecting a discrete Fourier transform by comparing the received pilot subcarrier information with the known pilot sequence.
The channel estimation based on the midamble data can be valid for the two adjacent symbols.
The two symbols together with the midamble can fit into one timeslot of the GSM standard.
The midamble can be used for frequency and time synchronization by means of a correlation technique.
The transmission method as set forth above can particularly be used for indoor applications or low-range outdoor applications where only a small guard time is necessary.
The present invention furthermore provides for a wireless transmission system comprising a transmitter and a receiver using an OFDM system comprising a plurality of equally spaced subcarriers. The transmission is effected in timeslots, wherein one timeslot comprises two OFDM-modulated symbols having the same time duration. A midamble is interposed between said two OFDM-modulated symbols, wherein the time duration of the midamble corresponds to 1/n of the time duration of the symbols, n being an integer greater than 1.
The transmitter can be adapted to transmit the midamble according to the OFDM system and the transmitter can comprise a modulator modulating only every n-th subcarrier when transmitting the midamble.
The transmitter can be adapted to transmit the midamble according to a single carrier system.
The transmitter can transmit pilot symbols in the midamble and the receiver can comprise a channel estimator for effecting a channel estimation for a coherent transmission based on the received pilot symbols.
The two symbols together with the midamble can fit in one timeslot of the GSM standard.
The receiver can comprise correlation means for frequency and time synchronization based on a correlation of samples of the midamble.
The present invention furthermore provides for a transmitter for the transmission of data according to an OFDM system. The transmitter thereby comprises a collector for collecting data for a first symbol of a timeslot, data for a midamble and data for a second symbol of the timeslot in this order. The time duration of the midamble thereby corresponds to 1/n of the time duration of the first and second symbol, respectively, n being an integer greater than 1. The collector is connected with an OFDM modulator modulating at least the first and the second symbols on a plurality of subcarriers according to the OFDM system.
The transmitter furthermore can comprise a pilot symbol generating unit outputting pilot symbols to an inverse Fourier transformation circuit, wherein the pilot symbols output from the pilot symbol generating unit are supplied to the collector as data for the midamble. The modulator modulates the pilot symbols in the midamble on every n-th subcarrier of an OFDM system.
Alternatively the transmitter can comprise a training sequence generating means outputting a training sequence directly to the collector as data for the midamble. The modulator in this case modulates the training sequence in the midamble according to a single carrier system.
According to the present invention furthermore a receiver for an OFDM system is provided comprising a channel estimator for a coherent detection of OFDM modulated symbols. The channel estimator is adapted to effect a channel estimation based on pilot symbols of a midamble being interposed between two succeeding OFDM-modulated symbols. The time duration of the midamble thereby equals 1/n of the time duration of the symbols, n being an integer greater than 1.
The receiver can comprise a correlation unit for effecting a time in frequency synchronization of the receiver. The correlation unit is adapted to correlate samples of a midamble interposed between two succeeding OFDM-modulated symbols. The time duration of the midamble thereby equals 1/n of the time duration of the symbols, n being an integer greater than 1.
The present invention will now be explained by means of different embodiments of the present invention and taken in reference to the figures of the annexed drawings.